New-fangled child seat requires both isofix (already got) and top-tether (which, in case it was news to you too, is basically: a strap like a seat belt which is supposed to pass over the top of the built-in seat - under the headrest I spose - down the back of same seat and hook onto a sturdy loop on the back).

Except no sturdy loop on mine, but there are little "hatches" on either side which pop out easy enough to reveal what seem like pre-tapped holes into the metal seat frame (see photos).

Reading up on a few forums and trawling through RAVE, the info seems pretty conflicting on what bracket it is you're supposed to fit, and how. Here's what I've ended up with by distilling the various bits of info:
- went for bracket P/N APU700010 (see photo)
- one of the guides says a spacer required, but the bracket looks to me like it's designed to bolt directly to the metal seat frame with just the hook poking through the little hatch
- use a 5/16" UNC bolt (correct thread - see photo)
- washer
- thread lock and tighten to 35Nm
- spring washer also? At this stage, I'm thinking no - thread lock should do the job

I wondered what those holes were for!
I should think threadlock will be fine, when you change your brake pads the carrier bolts come with it already on, so if it's good enough for your brakes....

Job done - parts and plan were spot-on, step-by-step photos attached, and here's the finished article ...

Well done, that looks very neat & "factory fit".